Diesel engines generally operate at a lower temperature and run leaner compared to conventional gasoline engines. While gasoline engines may put out more NOx on a ppm basis due to higher combustion temperatures, the three-way catalyst may reduce the NOx with high efficiency when the A/F ratio is at stoichiometry. Consequently, the NOx emissions from the catalyst-equipped gasoline engine can be lower than the NOx emissions from a diesel engine. Since diesels operate lean, the three-way catalyst may not reduce NOx. The reduction of NOx emissions is therefore a unique challenge for diesel engines. One approach to NOx reduction on diesel engines is the use of selective catalytic reduction (SCR) systems. SCR systems with urea injection may be very effective for reducing NOx under lean conditions once the temperature of the SCR catalyst is above a minimum operating level (e.g., 150° C. or 200° C.). However, before the SCR catalyst has reached this operating temperature, much of the NOx in the exhaust passes through the SCR catalyst without being reduced and is emitted from the vehicle.